timer: add new feature FixedRandomDelay=

FixedRandomDelay=yes will use
`siphash24(sd_id128_get_machine() || MANAGER_IS_SYSTEM(m) || getuid() || u->id)`,
where || is concatenation, instead of a random number to choose a value between
0 and RandomizedDelaySec= as the timer delay.
This essentially sets up a fixed, but seemingly random, offset for each timer
iteration rather than having a random offset recalculated each time it fires.

Closes #10355

Co-author: Anita Zhang <the.anitazha@gmail.com>

src/core/timer.h

@@ -59,6 +59,7 @@ struct Timer {
         bool remain_after_elapse;
         bool on_clock_change;
         bool on_timezone_change;
+        bool fixed_random_delay;
 
         char *stamp_path;
 };